The present invention relates generally to the field of electric distribution power systems and, more particularly, relates to a voltage sag and over-voltage compensation device for an AC electric power distribution system employing a pulse-width modulated transformer.
Voltage sags and over-voltage conditions occasionally occur on AC power distribution systems for a variety of reasons, such as high resistance faults in the distribution system, fault clearing, switching large loads characterized by arcing during connection or disconnection of the load, other types of transient circuit overloading (e.g., dynamic disturbances), high load inductance during unusually heaving load periods, and line capacitance during unusually light load periods. Although these voltage conditions may be short lived, such as a few cycles in a 50 or 60 Hertz electric power system for transient disturbances and fault clearing events, they can nonetheless cause sensitive loads, such as computer systems and manufacturing operations, to experience equipment damage and, in some cases, to drop off line. Therefore, devices that compensate for these voltage sags and over-voltage conditions, so that the loads receive an uninterrupted supply of the intended line voltage, serve an important function for these types of sensitive loads.
Certain conventional approaches to AC voltage compensation use traditional inverter technology, which rectifies the AC line power into DC power and stores the DC energy, typically in capacitors, batteries, or a flywheel during normal system operations. Then, during a voltage sag, the sag supporter device inverts the stored DC energy into AC power and delivers this power through a series-connected transformer to supply the missing voltage. This conventional inverter approach is complex and requires a large number of power switching elements to create the replacement voltage profile. The switching elements are relatively expensive and render the sag supporter financially infeasible for many applications. In addition, the duration of the available voltage support is limited by the amount of energy that can be stored prior to the voltage sag, and can therefore require large storage devices. Large storage devices can significantly increase the size of the device, often making pole-mounted configurations impractical. Alternatively, the circuitry required to repeatedly discharge and charge capacitors during the voltage sag condition presents complex control and timing challenges associated with continually recharging the capacitors to the proper level, and further increases the cost and sophistication of the device. In addition, the presence of the series-connected transformer in the power line during normal circuit operation causes significant power losses, even when voltage support is not required.
In another conventional approach, a tap switching series-connected transformer, often called a voltage regulator, may be used to compensate for voltage sag conditions. However, the large number of windings and switching elements required to provide a range of voltage sag compensation increases the cost of the voltage regulator and, in any event, limits the device to providing a discrete number of voltage steps in the output power supply. In addition, due the implementation time required for tap-changing voltage correction, these systems are ill suited to following fast changing voltage sag or over-voltage events, which typically occur when the cause of the voltage sag or over-voltage event involves a fault or switching event characterized by arcing. Arcing, by it""s very nature, is erratic in behavior and changes quickly during the event as attachment points move around. Again with this type of device, the presence of a series-connected transformer in the power line during normal operations causes significant power losses.
Transient over-voltage conditions caused by the tap switching series-connected transformer presents another significant disadvantage of the tap-switching voltage regulator approach. This typically occurs when a breaker or fuse clears a fault causing the voltage sag, which abruptly returns the system to normal voltage. This typically occurs at a zero-current condition, which is followed by the series-connected transformer boosting the voltage on its output well above its normal level for approximately 8 milliseconds until the transformer can be returned to its normal setting, which usually occurs at the next zero-current condition. This xe2x80x9ccurrent-zero switchingxe2x80x9d limitation occurs with these devices because they typically employ thyristor switching elements, which can only switch during zero-current conditions. Thus, notwithstanding multiple winding ratios and multiple switching elements, these systems still impose a significant over-voltage on the load at the conclusion of many voltage sag events.
Therefore, there is a need in the art for a compact, cost effective voltage sag and over-voltage compensation device that does not routinely impose over-voltage conditions on the loads they are designed to protect. There is also a need for a voltage sag and over-voltage compensation device that does not require a large number of switching devices, large power storage devices, or a series-connected transformer in the power line during normal operation of the circuit.
The present invention meets the needs described above in a voltage sag and over-voltage compensation device employing a pulse-width modulated transformer, which may be an autotransformer or a step-up transformer, such as a two-winding transformer. In various embodiments of the present invention, the modulating switch may be connected in series with the transformer, or the modulating switch may be connected in parallel with one winding of the transformer. All of these configurations may be operated in different modes to accomplish the objectives of the invention. In addition, the device preferably operates at AC electric power distribution system voltages, but may be designed to operate at other voltage levels.
The voltage sag and over-voltage compensation device employing a pulse-width modulated transformer significantly improves over conventional inverter technology in that no energy storage devices are required. The present invention also significantly improves over conventional tap-switching transformer technology in that no over-voltage is imposed on the load when a voltage sag event is over. Moreover, the technology of the present invention is much simpler than prior approaches for AC voltage compensation in that it reduces the number of active switching elements and uses well developed transformer technology as the basic design element. This advantageously allows switching to occur at higher voltages with lower currents than occurs in prior designs. In addition, a single switching frequency with a single pulse-width for any given voltage sag or over-voltage condition makes the control system for the present invention relatively simple to design and implement. The end result is a comparatively uncomplicated design, which exhibits lower cost and higher reliability, while providing equivalent or improved functionality in comparison to prior art voltage compensation technologies.
Generally described, the present invention includes a voltage sag and over-voltage compensation device that receives electric power from an AC power source oscillating at a system frequency, adjusts the voltage of the power, and delivers a corresponding voltage-corrected AC power supply to a connected load. The voltage sag and over-voltage compensation device includes a transformer and a modulating switch operable between an open configuration and a closed configuration for selectively connecting the AC power source to the transformer. The device also includes a control unit for selectively gating the modulating switch between the open configuration and the closed configuration multiple times per cycle of the system frequency to generate the voltage-corrected AC power supply for delivery to the load.
The modulating switch is typically located within a full-bridge rectifier circuit connected between the AC power source and the center pole to allow bi-directional switching through a single switching element (i.e., switching through the same element during the positive and negative portions of the AC voltage cycle). The voltage sag and over-voltage compensation device also typically includes a snubber connected in parallel with the modulating switch to absorb the current discharge caused by switching the power supply to the transformer under non-zero current conditions. In particular, the modulating switch and the snubber are typically connected in parallel and located within the full-bridge rectifier circuit connected between the AC power source and the center pole.
For relatively high voltage applications, the modulating switch includes a cascade of individual switching devices connected in series and operated substantially simultaneously, typically from a common gating signal. In this case, each individual switching device is located within an individual full-bridge rectifier circuit, and a plurality of snubber circuits may each be connected in parallel with one of the individual switching devices within its corresponding full-bridge rectifier circuit. The snubber circuits may include a resistor and a capacitor connected in series and a diode connected in parallel with the resistor. Alternatively, the snubber circuits may include a resistor and a capacitor connected in series without a diode connected in parallel with the resistor.
To smooth the voltage-corrected AC power supply toward a sinusoidal power supply at the system frequency, the voltage sag and over-voltage compensation device also includes a filter capacitor connected between the neutral and upper poles of the transformer. The control unit typically gates the modulating switch at a gating frequency, and to remove noise created by the switching elements, the voltage sag and over-voltage compensation device typically includes a notch filter connected between the neutral and upper poles or the transformer. This notch filter is preferably configured to reduce power disturbances occurring in the voltage-corrected AC power supply within a filter frequency range about the gating frequency. Specifically, the notch filter may include an inductor, a resistor, and a capacitor connected in series.
To permit normal operation of the power circuit without having the transformer connected in series in the power line, the voltage sag and over-voltage compensation device typically includes an upper-pole switch for selectively connecting the AC power source between the neutral and upper poles of the transformer when the upper-pole switch is gated to a closed or xe2x80x9conxe2x80x9d configuration, and for selectively disconnecting the AC power source from connection between the neutral and upper poles when the upper-pole switch is gated to an open or xe2x80x9coffxe2x80x9d configuration. To absorb the current discharge caused by switching the power supply to the transformer under non-zero current conditions, the voltage sag and over-voltage compensation device typically an upper-pole snubber connected in parallel with the upper-pole switch. For relatively high voltage applications, the upper-pole switch may include a cascade of individual: upper-pole switching devices connected in series and operated substantially simultaneously, typically from a common gating signal. In this case, each individual upper-pole switching device may be located within an individual full-bridge rectifier circuit, and a plurality of upper-pole snubber circuits may each be connected in parallel with one of the individual switching devices within its corresponding full-bridge rectifier circuit. Each upper-pole snubber circuit may include a resistor and a capacitor connected in series and a diode connected in parallel with the resistor.
In a first mode of operation, the control unit gates the upper-pole and switching devices in substantial unison to maintain these switches in opposing configurations. That is, the upper-pole switch is gated off when the modulating switch is gated on, and vice versa. More specifically, the control unit detects a voltage sag or over-voltage condition in the AC power source, and in response to detecting this condition, the control unit continually gates the upper-pole and switching devices in substantial unison to maintain these switches in opposing configurations to create a desired voltage-corrected AC power supply. The control unit then detects a cessation of the voltage sag or over-voltage condition in the AC power source, and in response to detecting the cessation of the condition, gates the switching device to an open configuration, gates the upper-pole switching device to a closed configuration, and discontinues further gating of the switching device.
In a second mode of operation, the control unit detects a voltage sag or over-voltage condition in the AC power source, and in response to detecting this condition, gates the upper-pole switching device to an open (i.e., by-pass) configuration. In this case, while the upper-pole switching device is held in the open (i.e., by-pass) configuration, the control unit continually gates the switching device between the open and closed configurations to create a desired voltage-corrected AC power supply. The control unit then detects a cessation of the voltage sag or over-voltage condition in the AC power source, and in response to detecting the cessation of the condition, gates the switching device to an open configuration, gates the upper-pole switching device to a closed configuration, and discontinues further gating of the switching device.
In view of the foregoing, it will be appreciated that the voltage sag and over-voltage compensation device of the present invention is a comparatively uncomplicated design, which exhibits lower cost and higher reliability, while providing equivalent or improved functionality in comparison to prior art technologies. The present invention also improves over prior AC voltage compensation technology in that no energy storage devices are required, and no over-voltage is imposed on the load at the conclusion of a voltage sag or event. The specific techniques and structures employed by the invention to improve over the drawbacks of the prior voltage sag and over-voltage compensation devices, and to accomplish the advantages described above will become apparent from the following detailed description of the embodiments of the invention and the appended drawings and claims.